1. Field of the Invention
The present invention relates to a liquid crystal display device functioning by transparency.
Such a device is used for displaying symbols and alpha numeric data resulting for example from measurements made by the instruments on board a motor vehicle nautical or aeronautical vehicles.
On board such a vehicle, it is desirable for the pilot to be able to acquaint himself with display information without ceasing to observe the environment visible through the windscreen, or cockpit, of his vehicle.
2. Description of the Prior Art
Liquid crystal display devices functioning by transparency are known comprising a liquid crystal wafer with control electrodes, normally transparent, whose symbol and character forming zones may become opaque when appropriate electric signals are applied to the control electrodes. Such a liquid crystal wafer, because it may transmit the light coming from behind the wafer or not is said to be of the "functioning by transmission" type. When such a wafer is disposed in the field of vision of the pilot, he then sees the symbols and characters to be displayed in black on a background formed by the surrounding landscape, for example. This device has however, the drawback of requiring a relatively luminous background and it does not allow displayed information to be read at night.
Other liquid crystal display devices functioning by transparency are also known, illuminated by an artificial light and therefore readable even at night. In these devices a liquid crystal wafer is used of a type comparable to the preceding type, that is to say functioning by transmission. An electric lamp for example is placed behind the wafer. So that the pilot is not dazzled by the light rays coming from the lamp and passing through the transparent zones of the wafer, between the wafer and the lamp is interposed a sheet of transparent material, provided with a multitude of opaque parallel and oblique flaps, which function as in a venetian blind for concealing the lamp from the pilot, while letting the oblique light rays pass through the flaps. These light rays illuminate the symbols and characters to be displayed, which diffuse the light and thus appear luminous against a dark background. A device of this type, visible at night, has however the serious drawback that, by day, by simply switching off the lamp, the ambient light cannot be used to cause the symbols and characters to appear over-printed on the landscape, because of the oblique flaps which cut off the substantially horizontal light rays coming from outside. Thus, no liquid crystal display device functioning by transparency is known allowing both daytime display by over-printing on the landscape and a display readable at night.
The present invention aims at providing a device of this type.